Method and apparatus for generating optical duo binary signals with frequency chirp

ABSTRACT

This present disclosure provides an optical transmission method and system. The system includes a pre-coder for pre-coding an input signal into a first pre-coded signal, an encoder/separator coupled to the first pre-coded signal and arranged to encode the first and second pre-coded signals into a first encoded signal with 0 degree phase shift and a second encoded signal with 180 degree phase shift, and an optical modulator for providing optical modulation to the first and second encoded signals with a light source such that the intensity of an output optical duo-binary (ODB) signal with frequency chirp has identical logic sequence as the input signal.

BACKGROUND 1. Field of the Invention

The present invention generally relates to an optical transmitter, andmore particularly to optical transmission method and apparatus forgenerating optical duo binary signals with frequency chirp. 2.Description of the Prior Art

In the information age, the demand for optical networks of higher datacapacities is constantly increasing. This demand is fueled by manydifferent factors, such as the tremendous growth of the Internet and theWorld Wide Web. Optical fiber transmission has played a key role inincreasing the bandwidth of telecommunications networks. It is thepreferred medium for transmission of data at high data rates and overlong distances nowadays.

At very high data rates, an inherent chromatic dispersion property,which means different spectral components of the signal traveling atdifferent speeds, in optical fiber transmission fibers causes waveformdeterioration and becomes a limiting factor in standard single-modefiber (SMF). Since there is a large installed base of SMF, a greatdemand for dispersion tolerant data transmission systems exists.

In standard optical communication systems, such as SONET, PDH, and SDH,data rates are in a hierarchy of 155 Mbps, 622 Mbps, 2.5 Gbps, and 10Gbps with a multiply factor of four. In SDH terminologies, the datarates above are referred as STM-1, STM-4, STM-16, and STM-64. Chromaticdispersion becomes important when data rate is counted in Gbpsmagnitude. Here, 10 Gbps data rate is taken as an example. In thisregards, the chromatic dispersion of a standard SMF is at 17 ps/nm*km at1550 nm. The spectral width of a chirp-free optical signal is given bythe Fourier transform limit, i.e., the width is equal approximately tothe inverse of the minimum pulse duration, or the data rate. Thus for aNRZ (non-return-to-zero) binary signal at 10 Gbps, the minimum pulseduration is 100 ps and the spectral width is around 10 GHz or 0.08 nm.After 70 km transmission in SMF, the signal pulses would be broadened byaround 100 ps, the minimum pulse duration or the bit period. Thereforethe dispersion limited transmission distance of a chirp-free 10 Gbps NRZoptical signal is about 70 km in SMF.

Various methods, such as ODB (Optical Duo-Binary) modulation, wereinvented to extend the dispersion limited distance. The characteristicof an ODB signal is that it has three phased modulated states, −1, 0,and +1 while it maintains two states of intensity, which follows theinput NRZ signal levels. There is no direct transition between −1 and +1state. Due to its property, ODB signal has narrower spectral width thanNRZ signal. Therefore, ODB signal could be transmitted farther indistance than NRZ signal in SMF.

A classical method of generating ODB signal was described in U.S. Pat.No. 5,543,952, “Optical Transmission System” by Yonenaga, et al. Athree-level electrical signal is generated by combining an input binarysignal with its exact 1-bit delayed replica. Then the three-levelelectrical signal is used to drive a MZ (Mach Zehnder) interferometertype modulator biased at null to generate the ODB signal. At 10 Gbpsdata rate, dispersion limited transmission distance of 120 km in SMF isobtained by applying the ODB signal. In addition, sharp cut-offfiltering of the driving signal is proposed by K. Yonenaga and S. Kuwanoin “Dispersion-Tolerant Optical Transmission System Using Duo-binaryTransmitter and Binary Receiver”, J. Lightwave Technol, Vol. 15, pp.1530-1537 (1997).

A partial response method of generating ODB signal was described in U.S.Pat. No. 5,867,534, “Optical Transmission Method with ReducedSensitivity to Dispersion, Transmission Device and System ForImplementing this Method” by Price, et al. A quasi three levelelectrical signal is first generated by passing an input NRZ signalthrough a narrow filter with bandwidth about 25%˜30% of the data rate.Then the three-level electrical signal is used to drive a MZ modulatorbiased at null to generate the ODB signal. At 10 Gbps data rate,dispersion limited transmission distance of 210 km in SMF is obtained byapplying the partial response method.

It is recognized that the longer transmission distance of an ODB signalis not entirely due to the narrower bandwidth. The “bumps” in theoptical signal near the “0” bit state also plays a very important rolein extending the transmission distance. The “bumps” are by-products ofboth partial response method and classical 1-bit delayed method usingsharp cut-off filters. Since the bumps have 180 degrees phase shift tothe adjacent “1” bit, the destructive interference between the bump andthe adjacent “1” bit would help to confine the broadening of the “1” bitpulse. Hence the dispersion penalty would be reduced accordingly. Due toits long transmission distance and its simple implementation, thepartial response method is the most widely used ODB method, and is oftenreferred as THE ODB method.

The “bumps” can be also introduced using a fractional delay, instead ofthe whole one-bit delay, in the classical ODB method. This method isdescribed in U.S. Pat. No. 6,623,188, “Dispersion Tolerant Optical DataTransmitter” by Dimmick, et al. A four-level electrical signal isgenerated by combining an input binary signal with its delayedcomplement through a differential amplifier in this disclosure. Thefour-level electrical signal is then used to drive a MZ modulator biasedat null to generate a four-level optical field, which could betransmitted of 150 km in SMF without any band-limiting filters at 10Gbps data rate.

Theoretically, combining an input binary signal with its delayedcomplement through a differential amplifier is equivalent to combiningthe input binary signal with its delayed replica. Within the frequencydomain, this is equivalent to passing the signal through a periodicalfilter with a frequency response, formulated as: $\begin{matrix}{{{Filter}\quad(f)} = \frac{1 + {\mathbb{e}}^{{{\mathbb{i}} \cdot 2 \cdot \pi \cdot f \cdot \Delta}\quad t}}{2}} & (1)\end{matrix}$where Δt is the time delay. In contrast, the partial response methodrequires low pass filters with smooth falling “tails” at highfrequencies. The combined effect of the Equation 1 and the intrinsicbandwidth of the driving circuit lead to the fractional delay methodgiving similar results as the partial response method.

Using frequency chirp for extending transmission distance was describedin U.S. Pat. No. 6,337,756, “Optical Transmitter System and Method”. Amethod to generate a classical 1-bit delayed ODB signal with frequencychirp was disclosed. Simulation shows that with small negative frequencychirp, transmission distance could be slightly improved over classicalODB.

Chirped ODB modulation was studied in “Chirped duo-binary transmissionfor mitigating the self-modulation limiting effect”, presented at theOFC 2001, March 2000 by M. Wicher, et al. It shows that negative chirpcould improve signal quality at shorter distance (<150 km at 10 Gbps)while positive chirp could improve signal quality at distances greaterthan 150 km, for example, a positively chirped signal was found totransmit greater than 250 km. In addition, the positive chirp was alsofound to reduce self phase modulation effect.

In the description of “Cost-effective optical chirped duo-binarytransmitter using an electro-absorption modulated laser”, IEEE PhotonTechnology Letters, Vol. 17, pp. 905-907, April 2005 by Hanlim Lee, etal; positively chirped ODB signals were generated with a combination ofelectro-absorption modulated laser (EML) and a MZ modulator. A similarresult was obtained by demonstrating that positively chirped ODB signalat 10 Gbps could be transmitted over 250 km in SMF at wavelength around1550 nm.

FIG. 1 is a schematic diagram illustrating a conventional ODBtransmitter 100. An electrical NRZ source 110 is fed into a pre-coder120, which is used to make the final optical output signal intensity ofthis transmitter 100 to be as the same as the input NRZ source 110. Inone case, the pre-coder 120 could be a simple XOR gate, with one inputbeing connected to the complement of the incoming NRZ data signal andthe other input being connected to the one-bit delayed XOR gate output.The output of the pre-coder 120 is sent into an ODB encoder 130, whichproduces a three-level or a four level signal ce(t) to a data driver140. As described earlier, the ODB encoder 130 could be implementedusing the delay-and-combine methods or using partial response method.The output of the ODB encoder 130 is amplified by the data driver andforwarded to drive a MZ modulator 150, which also receives opticalsource from a laser apparatus 160. The optical output of the MZmodulator is an ODB signal where the optical intensity I(t) follows theinput NRZ signal d(t).

The partial response method is the simplest one to generate chirp-freeODB signals with large dispersion tolerance. However, all chirp-free ODBsignals are very sensitive to non-linear distortion. For example, SPM(self phase modulation) on the transmitting signals can significantlyreduce their transmission distance because SPM makes a negativefrequency shift at the rising edge and a positive frequency shift at thefalling edge, which is generally referred to as a negative chirp.Coupled to the dispersion, the frequency domain distortion is convertedto time domain distortion, which is the source of non-linear penalty.

Therefore, a positive pre-chirp could be intentionally added to thetransmitting signal to compensate the SPM caused negative chirp in orderto reduce introduced non-linear penalty. The chirped ODB methods shouldhave better performance with higher signal launch power. However, themethods disclosed in prior art used double optical modulations, whichrequire precise time alignment between the electrical signals drivingthe two modulators. These two driving signals in the chirped ODB methodsare data patterns; one is NRZ signal and another is pre-coded data. Thevariable delay lines needed for the timing alignment have to haveuniform response over a broad bandwidth, which are usually bulky andexpensive. This is a big obstacle for the practical implementation ofthe chirped ODB modulation methods.

Thus there is a need for an improved and practical method to generatechirped ODB signals.

SUMMARY OF THE INVENTION

Therefore, in accordance with the previous summary, objects, featuresand advantages of the present disclosure will become apparent to oneskilled in the art from the subsequent description and the appendedclaims taken in conjunction with the accompanying drawings.

This present disclosure provides an optical transmission method andsystem. The system includes a pre-coder for pre-coding an input signalinto a pre-coded signal, an encoder/separator coupled to the pre-codedsignal and arranged to encode the pre-coded signal into a first encodedsignal with 0 degree phase shift and a second encoded signal with 180degree phase shift, and an optical modulator for providing opticalintensity modulation to the first and second encoded signals with anlight source such that the intensity of an output optical duo-binary(ODB) signal with frequency chirp has identical logic sequence as theinput signal.

The present invention offers longer transmission distance and bettertolerance to non-linear effects comparing to prior art. Furthermore, asingle modulator of the present invention is much simpler and lessexpensive than those methods using double optical modulations. It alsomakes practical implementation of chirped ODB signal possible.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings incorporated in and forming a part of thespecification illustrate several aspects of the present invention, andtogether with the description serve to explain the principles of thedisclosure. In the drawings:

FIG. 1 is a schematic diagram illustrating a conventional ODBtransmitter;

FIG. 2 is a schematic diagram of a chirped ODB transmitter according toone embodiment of the present invention;

FIG. 3A is a schematic diagram of the pre-coder and the ODBencoder/bit-separator shown in FIG. 2;

FIG. 3B is a timing diagram illustrating signals in each stage of theembodiment shown in FIG. 3A;

FIG. 4A is another schematic diagram of the pre-coder and the ODBencoder/bit-separator shown in FIG. 2;

FIG. 4B is a timing diagram illustrating signals in each stage of theembodiment shown in FIG. 4A;

FIG. 5A is another schematic diagram of the pre-coder and the ODBencoder/bit-separator shown in FIG. 2;

FIG. 5B is a timing diagram illustrating signals in each stage of theembodiment shown in FIG. 5A;

FIGS. 6A and 6B are two schematic diagrams of the pre-coder and the ODBencoder/bit-separator shown in FIG. 2; and

FIG. 7 is a diagram showing signal distortions of a positive chirped, anegative chirped, and a chirp free 10 Gbps ODB signals at variousdistances.

DETAILED DESCRIPTION

Reference will now be made in detail as to the description of theinvention as illustrated in the drawings. While the invention will bedescribed in connection with these drawings, there is no intent to limitit to the embodiments disclosed therein. On the contrary, the intent isto cover all alternatives, modifications and equivalents included withinthe spirit and scope of the invention as defined by the appended claims.

The present disclosure describes several embodiments given below. It isunderstood, however, that the embodiments below are not necessarilylimitations to the present disclosure, but are used as typicalimplementations of the invention. It will be appreciated from thedescription provided herein that a variety of alternative embodimentsand implementations may be realized, consistent with the scope andspirit of the present invention.

FIG. 2 is a schematic diagram of a chirped ODB transmitter 200 inaccordance with one embodiment of the present invention. An electricalNRZ source 210 provides an input d(t) to be fed into a pre-coder 220.Both two complementary outputs c(t) 221 and c(t) 222 of the pre-coder220 are sent to an integrated ODB encoder/bit-separator 230, whichperforms a predetermined encoding process and then separates the bitsintended for each optical phase in an ODB modulation. After separation,those bits, intended for 0 degree phase shift, are sent to one output ofthis integrated ODB encoder/bit-separator 230. On the other hand, theother bits, intended for 180 degree phase shift are sent to anotheroutput of the integrated ODB encoder/bit-separator 230. In one example,these two outputs are denoted as a first output c1(t) 231 and a secondoutput c2(t) 232, respectively. Then the two separated sequences, c1(t)231 and c2(t) 232, are amplified by a first and a second data drivers241 and 242 to drive two separate inputs of a dual drive MZ modulator250, respectively. The MZ modulator 250, receiving optical source from alaser apparatus 260, generates an ODB signal I(t) 251 with frequencychirp. The sign of the frequency chirp is determined by choosing eitherC1(t) and C2(t) signal, or the complements, C1(t) and C2(t), as thedriving signals.

FIG. 3A is a schematic diagram of the pre-coder 220 and the ODBencoder/bit-separator 230 shown in FIG. 2. In this embodiment, thepre-coder 220 includes an XOR gate 223 with an input receiving d(t) fromthe NRZ source 210. The other input of the XOR gate receives a one-bitdelayed output of this XOR gate 223. The output of the XOR gate 223 isreferenced as the output signal c(t) 221. On the other hand, thecomplementary output signal c(t) 222 is generated by an inverter 225from the output of the XOR gate 223.

The ODB encoder/bit-separator 230 comprises a first and a second ANDgates 233 and 234. The first AND gate 233 takes the output signal c(t)221 and its delayed replicated signal c(t−Δt) as inputs in order togenerate the first output c1(t) 231. Similarly, the second AND gate 234takes the complementary output signal c(t) 222 and its delayedreplicated signal c(t−Δt) as inputs in order to generate the secondoutput c2(t) 232. In one example of this embodiment, the time delay, At,can be ranged from 0.3 to 1 bit period.

FIG. 3B is a timing diagram illustrating signals in each stage of theembodiment shown in FIGS. 2 and 3A. In this diagram, there are sixstaged timing sequences, the NRZ input signal d(t) 210, c(t) 221,c(t−Δt), c1(t) 231, c2(t) 232, and the final output I(t) 251. It isassumed that the time delay, Δt, is 0.7-bit period. Some narrow pulseswith 0.3-bit duration could be observed in c1(t) 231 and c2(t) 232accordingly. The finite response of the circuit, which is at 75% of datarate, lowered the amplitude of these narrow pulses, forming the “bumps”.At the bottom, the timing sequence of I(t) 251 is the optical output ofthe dual drive MZ modulator 250.

FIG. 4A is another schematic diagram of the pre-coder 220 and the ODBencoder/bit-separator 230 shown in FIG. 2. Similar to FIG. 3A, twocomplementary outputs c(t) 221 and c(t) 222 of the pre-coder 220 aresent into the ODB encoder/bit-separator 230 as inputs. However, in thisembodiment, the ODB encoder/bit-separator 230 comprises a first and asecond LIA (differential limiting amplifiers) 235 and 236 instead of thetwo AND gates 233 and 234 shown in FIG. 3A. The basic operationalfunction of an LIA is that when the total input signal level is higherthan a specified threshold level, the binary output of the LIA would beswitched to logic “high”; otherwise, the binary output would be logic“low”. With a proper set threshold level, either the positive or thenegative pulses from the differential input signals could be filtered orselected. The first LIA 235 takes the output c(t) 221 and the delayedcomplementary output c(t) 222 as inputs. In parallel, the second LIA 236takes the delayed output c(t) 221 and the complementary output c(t) 222as inputs. Similar to the embodiment shown in FIG. 3A, the time delay,Δt, in this embodiment can range from 0.3 to 1 bit period. Depending onthe length of the time delay, the differential input signals of thesetwo LIA 235 and 236, c(t)− c(t−Δt) and c(t−Δt)− c(t), could bethree-level or four-level signals. The differential input signals aretreated as AC signals, which swing from −1 to +1. Given a properpositive threshold, the output of the first LIA 235 could besubstantially similar to the output of the first AND gate 233 shown inFIG. 3A. On the other hand, given a proper negative threshold, thecomplementary output of the second LIA 236 could be substantiallysimilar to the output of the second AND gate 234 shown in FIG. 3A.

FIG. 4B is a timing diagram illustrating signals in each stage of theembodiment shown in FIG. 4A. In this diagram, there are six timingsequences, the NRZ input signal d(t) 210, c(t) 221, c(t)− c(t−Δt), c1(t)231, c2(t) 232, and the final output I(t) 251. Except for the thirdtiming sequence c(t)− c(t−Δt), the rest of five sequences are also shownin FIG. 3B. It is assumed that the time delay, Δt, is 0.7-bit period inthis embodiment. In addition, a threshold level 410 of the first LIA 235is set at +0.5 by assuming that c(t)− c(t−Δt) swings between −1 and +1.All signals above this threshold level line 410 are kept by the firstLIA 235 as the fourth timing sequence, c1(t) 231. On the other hand, athreshold level (not shown) of the second LIA 236 could be also set at−0.5 by assuming that c(t−Δt)− c(t) ranges between −1 and +1. As aresult, the output of this second LIA 236 is shown as the fifth timingsequence, c2(t) 232. The fourth and fifth timing sequences of FIG. 4Bresemble the counterparts shown in FIG. 3B. At the bottom, the timingsequence, I(t) 251, is the optical output of the dual drive MZ modulator250.

FIG. 5A is another schematic diagram of the pre-coder 220 and the ODBencoder/bit-separator 230 shown in FIG. 2. Unlike the embodiments shownin the previous FIGS. 3A and 4A, only the output signal c(t) 221 is sentto the ODB encoder/bit-separator 230, and its complementary one isgrounded. In this embodiment, the ODB encoder/bit-separator 230 furthercomprises a LPF (low-pass-filter) 237 and the first and second LIA 235and 236. In one example, the bandwidth of the LPF 237 could be setbetween 25% and 65% of data rate. Thus the output signal cf(t) 238 ofthe LPF 237 would be a four-level signal, with shorter pulses havinglower amplitudes than those in the conventional art. The output signalcf(t) 238 is treated as an AC signal, which swings in the range from−0.5 to +0.5, and splits into the one input of the first LIA 235 andanother input for the second LIA 236. Given a properly set thresholdlevel, the output of the first LIA 235 could be substantially similar tothe output of the first AND gate 231. Similarly, the output of thesecond LIA 236 could be substantially similar to the output of thesecond AND gate 232 shown in FIG. 3A.

FIG. 5B is a timing diagram illustrating signals in each stage of theschematic shown in FIG. 5A. In this diagram, there are six timingsequences, the NRZ input signal d(t) 210, c(t) 221, cf(t) 238, c1(t)231, c2(t) 232, and the final output I(t) 251. Except for the thirdtiming sequence cf(t) 238, the rest of five sequences are similar tothose shown in FIG. 3B and 4B. It is assumed that the bandwidth of theLPF 237 is set to be 50% of data rate and the magnitude of the signal isexpanded by a factor of 2 for illustration. A dash line 510 is used tomark the threshold level, +0.33, for the first and second LIA 235 and236, by assuming that the signal, cf(t) 238, swings in the range of −0.5to +0.5. At the bottom, the timing sequence, I(t) 251, is the opticaloutput of the dual drive MZ modulator 250.

FIG. 6A is another schematic diagram of the pre-coder 220 and the ODBencoder/bit-separator 230 shown in FIG. 2. In this embodiment, theencoder/bit-separator 230 comprises a first and a second LPF 601 and602, that receive the complementary outputs c(t) 221 and c(t) 222 asinputs, respectively. In one example, the bandwidth of both LPF 601 and602 can be set between 25% and 65% of the data rate. Both of the outputsignals cf(t) 603 and cf(t) 604 are four-level signals and treated as ACsignals, swinging in the range from −0.5 to +0.5. Both cf(t) 603 andcf(t) 604 are sent to the first and second LIA 235 and 236, which have acommon positive threshold, respectively. As a result, the output of thefirst LIA 235 could be substantially similar to the output of the firstAND gate 231. Similarly, the output of the second LIA 236 could besubstantially similar to the output of the second AND gate 232 shown inFIG. 3A. When the bandwidth of the first and second LIA 235 and 236 areset to be 50% of the data rate and the common threshold is set at +0.33,the resulting timing sequences diagram of this embodiment would besubstantially similar to that in FIG. 5B.

FIG. 6B is yet another schematic diagram of the pre-coder 220 and theODB encoder/bit-separator 230 shown in FIG. 2. This embodiment isessentially identical to that shown in FIG. 6A, except that the two LPF601 and 602 are substituded by two unbalanced MZ interferometer filters.An unbalanced MZ interferometer filter is formed by first splitting aninput signal to two signals, with each passing a path with unequallength, and then recombining the two splitted signals. The unequallength causes a relative delay, Δt, between the signals. Similar to theembodiment shown in FIG. 3A, the time delay, Δt, in this embodiment canrange from 0.3 to 1 bit period. As a result, the output of the first LIA235 could be substantially similar to the output of the first AND gate231. Similarly, the output of the second LIA 236 could be substantiallysimilar to the output of the second AND gate 232 shown in FIG. 3A. WhenΔt is set to 0.7 bit period, the resulting timing sequences diagram ofthis embodiment would be substantially similar to that in FIG. 4B.

Please refer to FIG. 7, which contains diagrams a) to m) for showingsignal distortions of a positive chirped, a negative chirped, and achirp free 10 Gbps ODB signals at various distances. The four columnsshow eye diagrams at 0 km, 100 km, 200 km, and 250 km from left toright. The three rows show eye diagrams of the positive chirped, thenegative chirped, and the chirp free ODB signals. If the driving voltagefor the MZ modulator 250 is 0.5 VΠ, the positive chirped (from a) to d))and negative chirped (from e) to h)) ODB signals are generated accordingto the embodiments shown in FIG. 2, using AND gates for illustrationpurpose with the understanding that the results of the rest threeembodiments are substantially similar to these two rows. It is clearfrom observing these eye diagrams to conclude that the positive chirpedODB signal could reduce signal distortion at longer distances.

In summary, the present invention offers longer transmission distanceand better tolerance to non-linear effects than conventional art.Furthermore, using a single modulator is much simpler and economicalthan those conventional methods using double optical modulations. Italso makes practical implementation of chirped ODB signals possible.

The foregoing description is not intended to be exhaustive or to limitthe invention to the precise forms disclosed. Obvious modifications orvariations are possible in light of the above teachings. In this regard,the embodiment or embodiments discussed were chosen and described toprovide the best illustration of the principles of the invention and itspractical application to enable one of ordinary skilled in the art toutilize the invention in various embodiments and with variousmodifications as are suited to the particular use contemplated. All suchmodifications and variations are within the scope of the inventions asdetermined by the appended claims.

1. An optical transmitter system comprising: a pre-coder for pre-codingan input signal into a pre-coded signal; an encoder/separator coupled tothe pre-coded signal and arranged to encode the pre-coded signal into afirst encoded signal with 0 degree phase shift and a second encodedsignal with 180 degree phase shift; and an optical modulator forproviding optical modulation to the first and second encoded signalswith a light source such that the intensity of an output opticalduo-binary (ODB) signal with frequency chirp has identical logicsequence as the input signal.
 2. An optical transmitter system of claim1, wherein the optical modulator comprising a dual drive Mach-Zehndermodulator for providing optical modulation based on the first and secondencoded signals.
 3. An optical transmitter system of claim 1, whereinthe pre-coder further comprises an XOR gate receiving a first input fromthe complement of the input signal, and a second input from a delayedoutput of the XOR gate, wherein the XOR gate provides the output of itas the pre-coded signal.
 4. An optical transmitter system of claim 1,wherein the encoder/separator further comprises: a first AND gate forgenerating the first encoded signal based on the first pre-coded signaland a delayed first pre-coded signal; and a second AND gate forgenerating the second encoded signal based on the complement of thepre-coded signal and a delayed complement of pre-coded signal.
 5. Anoptical transmitter system of claim 4, wherein a delay time of thedelayed first pre-coded signal and the delayed complement of pre-codedsignal is the same.
 6. An optical transmitter system of claim 1, whereinthe encoder/separator further comprises: a first differential limitingamplifier (LIA) for generating the first encoded signal based on thepre-coded signal and a delayed complement of the pre-coded signal; and asecond LIA for generating the second encoded signal based on thecomplement of the pre-coded signal and a delayed pre-coded signal.
 7. Anoptical transmitter system of claim 6, wherein a delay time of thedelayed pre-coded signal and the delayed complement of the pre-codedsignal is the same.
 8. An optical transmitter system of claim 1, whereinthe encoder/separator further comprises: a low pass filter (LPF) forfiltering the pre-coded signal; a first differential limiting amplifier(LIA) for receiving an output of the LPF, and generating the firstencoded signal using a first predetermined threshold; and a second LIAfor receiving the output of the LPF and generating the second encodedsignal using a second predetermined threshold.
 9. An optical transmittersystem of claim 8, wherein the output of the LPF is a four-level signal.10. An optical transmitter system of claim 8, wherein the bandwidth ofthe LPF ranges between 25% and 65% of data rate of the input signal. 11.An optical transmitter system of claim 1, wherein the encoder/separatorfurther comprises: a first low pass filter (LPF) for filtering thepre-coded signal; a second LPF for filtering the complement of thepre-coded signal; a first differential limiting amplifier (LIA) forreceiving an output of the first LPF and generating the first encodedsignal using a first threshold; and a second LIA for receiving an outputof the second LPF and generating the second encoded signal using asecond threshold.
 12. An optical transmitter system of claim 11, whereinthe outputs of the first and second LPF are four-level signals.
 13. Anoptical transmitter system of claim 11, wherein the bandwidth of thefirst and second LPF ranges between 25% and 65% of data rate of theinput signal.
 14. An optical transmitter system of claim 1, wherein theencoder/separator further comprises: a first unbalanced MZinterferometer filters for filtering the pre-coded signal; a secondunbalanced MZ interferometer filters for filtering a complement of thepre-coded signal; a first differential limiting amplifier (LIA) forreceiving an output of the first LPF and generating the first encodedsignal using a first threshold; and a second LIA for receiving an outputof the second LPF and generating the second encoded signal using asecond threshold.
 15. An optical transmitter system of claim 14, whereinthe first and second unbalanced MZ interferometer filters introduce adelayed path for the pre-coded signal and the complement of thepre-coded signal to be split and combined.
 16. A method for generatingoptically duo-binary (ODB) signals with frequency chirp in an opticaltransmission system, comprising: pre-coding an input signal into apre-coded signal; encoding the pre-coded signal into a first encodedsignal with 0 degree phase shift and a second encoded signal with 180degree phase shift; and modulating a light source such that an intensityof an output ODB (optical duo-binary) signal with frequency chirp hasidentical logic sequence as the input signal.
 17. The method of claim16, wherein the modulating further comprises using a dual driveMach-Zehnder modulator based on the first and second encoded signals.18. The method of claim 16, wherein the pre-coding further comprisesperforming a logic XOR operation with the complement of the input signaland a one-bit delayed output of the XOR gate, wherein the pre-codedsignal is the output of the XOR operation.
 19. The method of claim 16,wherein the encoding further comprises: performing a first logic ANDoperation with the pre-coded signal and a delayed pre-coded signal; andperforming a second logic AND operation the complement of the pre-codedsignal and a delayed complement of the pre-coded signal, wherein anoutput of the first AND operation is the first encoded signal and anoutput of the second AND operation is the second encoded signal.
 20. Themethod of claim 19, wherein a delay time of the delayed pre-coded signaland the delayed second pre-coded signal is from 0.3 to 1 bit period. 21.The method of claim 16, where the encoding further comprises: using afirst differential limiting amplifier for amplifying the pre-codedsignal and a delayed complement of the pre-coded signal; and using asecond differential limiting amplifier for amplifying the complement ofthe pre-coded signal and a delayed pre-coded signal, wherein an outputof the first differential limiting amplifier with a first threshold isthe first encoded signal and an output of the second differentiallimiting amplifier with a second threshold is the second encoded signal.22. The method of claim 21, wherein a delay time of the delayedpre-coded signal and the delayed second pre-coded signal is from 0.3 to1 bit period.
 23. The method of claim 16, wherein the encoding furthercomprises: a low-pass filtering of the pre-coded signal; a firstdifferential limiting amplifying of an output of the low pass filtering;and a second differential limiting amplifying the inversed output of thelow pass filtering, wherein an output of the first differential limitingamplifying with a first threshold is the first encoded signal and anoutput of the second differential limiting amplifying with a secondthreshold is the second encoded signal.
 24. The method of claim 23,wherein the bandwidth of the filtering is ranged between 25% and 65% ofdata rate of the input signal.
 25. The method of claim 16, wherein theencoding further comprises: a first low pass filtering of the pre-codedsignal; a second low pass filtering of the complement of the pre-codedsignal; a first differential limiting amplifying of an output of thefirst low pass filtering; and a second differential limiting amplifyingof an output of the second low pass filtering, wherein an output of thefirst differential limiting amplifying with a first threshold is thefirst encoded signal and an output of the second differential limitingamplifying with a second threshold is the second encoded signal.
 26. Anoptical transmitter system comprising: a pre-coder for pre-coding aninput signal into a pre-coded signal; an encoder/separator coupled tothe pre-coded signal and arranged to encode the pre-coded signal into afirst encoded signal with 0 degree phase shift and a second encodedsignal with 180 degree phase shift, the encoder further comprising: afirst AND gate for generating the first encoded signal based on thefirst pre-coded signal and a delayed first pre-coded signal; and asecond AND gate for generating the second encoded signal based on thecomplement of the pre-coded signal and a delayed complement of thepre-coded signal; and an optical modulator for providing opticalmodulation to the first and second encoded signals with a light sourcesuch that the intensity of an output optical duo-binary (ODB) signalwith frequency chirp has identical logic sequence as the input signal.27. An optical transmitter system of claim 26, wherein the pre-coderfurther comprises an XOR gate receiving a first input from thecomplement of the input signal, and a second input from a delayed outputof the XOR gate, wherein the XOR gate provides the output of it as thepre-coded signal.
 28. An optical transmitter system of claim 26, whereina delay time of the delayed pre-coded signal and the delayed complementof pre-coded signal is the same.